How do I make a transition between two values smoothly?

So here is the deal:

I have a Global Variable with a number, and when my the player press a button, the number becomes X + 40. The thing is, I don't want to make the change instantaneously, I want the value to change smoothly during a set period of time. How do I do it?

I'm really lost here.

https://www.dropbox.com/s/eqkfr83ng5t95 ... .capx?dl=0

Perfect. Just perfect. Thank you very much!